There are three types of "blooming" which occur in television systems. One type is "spot" blooming, which results from the brightness and contrast controls of the television receiver or monitor being improperly set such that the receiver or monitor does not operate within the linear region of the camera.
Another type of blooming, associated with receivers, is caused by atmospheric conditions or man-made disturbances, usually of a temporary nature.
The present invention concerns the third type of blooming, and this type is caused by bright lights within the televised scene. Blooming of this type results from the inability of the electron beam in the camera to neutralize completely the affected area of the target. When this type of blooming effect occurs, the scene as viewed on a monitor appears to be washed out. Further, when bright areas appear within the scene, they are very distracting to the viewer. The net result in both cases is a loss of information within and adjacent to the affected areas.
Various mechanical methods have been used in the prior art to overcome the scene-related type of blooming. For example, one of these methods involves the use of a motorized iris within the lens, while another method uses a motorized filter wheel. Such mechanical methods have a major disadvantage in that, although they do attenuate the bright areas, they also attenuate the scene around the affected areas.
Electronic circuits have also been employed in the prior art to compensate for the various types of blooming noted above. Examples of such circuits are disclosed in U.S. Pat. Nos. 2,414,228 (Gottier); 2,978,537 (Kruse et al.); and 3,179,743 (Ahrons). The Gottier circuit employs a blocking diode to clip the white peaks of the video signal above a preset level to reduce blooming. A disadvantage of this type of circuit is that any information within the bright area is completely lost. Although this loss of information can be tolerated in a home television receiver, such loss cannot be tolerated in more demanding applications, such as, for example, a military environment.
The Kruse et al. circuit uses an averaging technique to produce a direct current (D.C.) voltage, the amplitude of which is an inverse function of the average peak-to-peak value of the video signal over a pre-determined time period, for controlling or serving as the supply voltage for one or more of the camera tube electrodes. This method of control has the disadvantage of affecting the overall picture and not just the high peaks of the signal which produce the blooming effect. The Kruse et al. method of control has the further disadvantage of introducing a time delay in the circuit because of the closed loop configuration which is employed.
The Ahrons circuit is directed to control of spot blooming resulting from the contrast control being set too high, and employs for this purpose two diodes by which the white peaks which would cause blooming are separated from the rest of the video signal. The separated white peak signals are then amplified to provide a D.C. control signal which is used to control the AGC voltage so as to decrease the overall set gain. The Ahrons circuit thus suffers from the same disadvantage as does the Kruse et al. circuit.
Electronic circuits are also known in the prior art which compensate for noise pulses. An example of such a circuit as employed in a television receiver is disclosed in U.S. Pat. No. 2,424,349 (Cawein). The Cawein circuit eliminates random rate noise pulses resulting from atmospheric or man-made disturbances which can cause instability in the video and sync circuits of a receiver. This purpose is achieved in the Cawein circuit by demodulating the carrier wave to separate out noise currents having intensities in excess of a predetermined level, inverting the demodulated noise signal, and combining the inverted signal with the carrier wave to eliminate the demodulated noise signals therefrom. The Cawein circuit would be inappropriate for use in controlling scene-related blooming, since any information in the demodulated signals representing the bright areas would be lost by the cancelling effect produced by the Cawein circuit.